Almost four million pieces of malware were detected on 535,752 machines in the same period, according to a Microsoft blog post.
The company's Security Essentials software provides basic anti-virus protection to licensed users of the Windows operating system not covered by other security software.
"In one week, Microsoft Security Essentials is making a big difference to those people using it on their computers," the blog post said.
The free software has proved most popular with business users of the Windows 7 operating system, which is due for consumer release later this week.
According to Microsoft, 44% of Security Essentials users are running Windows 7, followed by XP (33 %) and Vista (23%).
Fifty two percent of malware detected in the first week was found on machines running XP, followed by Vista (32%) and Windows 7 (16%).
"This follows our usual observed trend of seeing less malware on newer OS and service packs," the blog post said.